Acrylic rubbers have good cold resistance and good oil resistance, and therefore, are widely used as rubber parts, which are placed in contact with a metal element or oil, such as a sealing material, a hose material, a vibration insulator, a tubular material, a belt material and a boot material. It is desired that the acrylic rubbers have a reduced permanent set as well as good heat resistance and cold resistance. Further, it is desired to impart to the acrylic rubbers excellent resistance to metal corrosion and oil resistance as well as the above-mentioned properties.
Acrylic rubbers have a problem such that they have a short scorch time and are readily subject to scorch. There is an increasing demand for extending the scorch time and reducing the tendency of scorching.
Further, when unvulcanized acrylic rubbers are kneaded, they readily stick to a metal surface inside a kneader such as a Banbury mixer or a roll mill. Therefore, cleaning of a kneader is indispensable after kneading is conducted. It is requested to reduce the stickiness of acrylic rubbers to a metal surface.
As an acrylic rubber material exhibiting a long scorch time, an ethylene-acrylic acid ester-butenedioic acid monoester copolymer has been proposed in Japanese Unexamined Patent Publication (hereinafter abbreviated to as “JP-A”) No. S50-45031. However, this acrylic rubber material is said to have poor oil resistance (JP-A H11-92614).
As a vulcanizate exhibiting good resistance to metal corrosion and having good oil resistance, a vulcanizate made from a vulcanizable rubber composition comprising an acrylic rubber having copolymerized with a fumaric acid mono-lower alkyl ester, an aromatic diamine vulcanizer and a guanidine vulcanization accelerator has been proposed in JP-A H11-92614. This vulcanizable acrylic rubber composition has a reduced tendency of scorching, but tends to exhibit stickiness to a metal surface.
To reduce the stickiness of an acrylic rubber to a metal surface, it is adopted to add an internal release agent such as an ester-type wax, a paraffin wax, a metal salt of an organic carboxylic acid, or silicone oil, in an acrylic rubber. However, a vulcanizate, made from the acrylic rubber having added thereto the internal release agent, has undesirably large permanent set and poor physical properties.